Ensure dependabot config exists and is correct

## Summary

Per [the plan to make this org's add-ons more consistent so it is simpler for org volunteers to maintain](https://github.com/adopted-ember-addons/program-guidelines/issues/28), we want to maintain the addons using dependabot across all repos where reasonable.

## Outcome

Dependabot is configured correctly for the repo.

## Acceptance Criteria

This issue is considered complete when:

- [ ] There is a dependabot config file in the `.github` folder
- [ ] the config file is consistent with other adopted addons
- [ ] Issues from any other maintenance bots are closed
- [ ] config files for any other maintenance bots are removed

Questions? Ask in the `#adopted-ember-addons` channel in Discord.